The Galactic Halo Contains Outer and Inner Shells of Stars

The galactic halo contains outer and inner shells of stars. How does the outer shell compare to the inner shell?

A) The two shells are virtually identical except for their distance from the galactic nucleus.
B) The stars of the outer shell are much younger than those of the shell layer.
C) The stars of the outer shell are relatively metal-rich.
D) The stars of the outer shell orbit in the opposite sense from those of the inner shell.